1. Field of the Invention
The present invention relates to a composite electrode which has a wide application in various electrochemical devices such as batteries, electrochromic display devices, sensors and memories. More particularly, the present invention provides a composite electrode which comprises a plurality of organic compounds, or a plurality of organic and inorganic compounds. It is further concerned with a method for producing such an electrode as well as a lithium secondary battery using such an electrode as its cathode.
2. Description of the Related Art
Since conductive polyacetylene was discovered by Shirakawa et al. in 1971, the use of this conductive polymer as an electrode material has been extensively studied. This is because electrochemical devices such as lightweight batteries with a high energy density, electrochromic display devices having a large area and rapid coloring and decoloring characteristics, and biochemical sensors using minute electrodes can be expected to be realized by using the conductive polymer as the electrode material. The polyacetylene however has some disadvantages for practical use in the electrodes because of its chemical instability, and hence, research has since been directed to other .pi. electron conjugated conductive polymers, which are relatively stable, such as polyaniline, polypyrrole, polyacene and polythiophene. Lithim secondary batteries using these polymers for their cathodes have already been developed.
Separate from this, disulfide compounds have been proposed as organic materials which may realize a high energy density in electrochemical devices, for example, disclosed in Dejoughe et al., (U.S. Pat. No. 4,833,048). Typical disulfide compounds are represented, in their most basic form, by the formula: R--S--S--R (wherein, R represents an aliphatic or an aromatic organic residue, and S represents a sulfur atom). The S--S bond may be cleaved by electrolytic reduction to form a salt with a cation (M.sup.+) in an electrolytic cell. The salt is represented by the formula of R--S.sup.- .multidot.M.sup.+. The salt of R--S.sup.- .multidot.M.sup.+ may be returned to the original R--S--S--R by electrolytic oxidation. A metal-sulfur type rechargeable battery constructed by combining a disulfide type compound with a metal (M) which supplies and captures cations (M.sup.+) is proposed in Dejoughe et al. As described therein, it is expected that such a secondary battery will have an energy density of 150 Wh/kg or more, which is much more than that of the conventional secondary batteries.
These conductive polymer electrodes, comprising at least one of polyaniline, polypyrrole, polyacene and polythiophene, take in anions existing in the electrolyte as well as cations upon an electrode reaction while they are charged or discharged, and hence the electrolyte in the battery not only functions as a transfer medium for ion conduction but also participates in the battery reaction. Therefore, it is required that the electrolyte is supplied to the battery in an amount corresponding to the battery capacity. Thus, it has some disadvantages that the energy density of the battery is limited that much to the range of about 20 to 50 Wh/kg, which is nearly half of that of the conventional secondary batteries such as nickel-cadmium or lead-acid batteries.
On the other hand, as Dejoughe et al. reported in J. Electrochem. Soc., Vol. 136, No. 9, pp. 2570 to 2575 (1989), the difference between oxidation potential and reduction potential of the organic disulfide compound is as large as 1 volt or more in an electrolysis of, for instance, a compound [(C.sub.2 H.sub.5).sub.2 NCSS.sup.- ].sub.2. According to the electrode reaction theory, the electron transfer of the disulfide compound proceeds extremely slowly. Because of this, it is rather difficult to obtain a rechargeable battery which may provide a higher current output of, for instance, 1 mA/cm.sup.2 or more at room temperature. Therefore, there is a disadvantage that the operation of a battery comprising an electrode of disulfide compound is limited to that at a high temperature in the range of from 100.degree. C. to 200.degree. C., where the electron transfer can proceed faster.